SIPROTEC 4 7SA522 Distance Protection Relay for Transmission ...

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SIPROTEC 4 – 7SA522 Distance Protection Relay for Transmission Lines Functions Distance protection (ANSI 21, 21N) Full-scheme design The main function of the 7SA522 is a full-scheme distance protection. By parallel calculation and monitoring of all six impedance loops a high degree of sensitivity and selectivity is achieved for all types of faults. The shortest tripping time is less than one cycle. Single- and three- pole tripping is possible. MHO and quadrilateral characteristics The 7SA522 relay provides quadrilateral as well as MHO zone characteristics. Both characteristics can be used separately for phase and ground faults. Resistance ground faults can for instance be covered with the quadrilateral characteristic and phase faults with the mho characteristic. Load bias In order to guarantee a reliable discrimination between load operation and short circuit – especially on long high loaded lines – the relay is equipped with a selectable load encroachment characteristic. Impedances within this load encroachment characteristic will prevent the distance zones from unwanted tripping. Absolute phase-selectivity The 7SA522 distance protection incorporates a well proven highly sophisticated phase selection algorithm. The pickup of unfaulted loops is eliminated to prevent the adverse influence of currents and voltages in the fault free loops. This phase selection algorithm achieves single pole tripping and correct distance measurement in a wide application range. Parallel line compensation The influence of wrong distance measurement due to parallel lines can be compensated by feeding the neutral current of the parallel line to the relay. Parallel line compensation can be used for distance protection as well as for the fault locator. 6 zone packages Five independent distance zones and one separate overreach zone for zone extension schemes are available. Each distance zone has separate time stages, partly valid for single phase or multi-phase faults. Ground faults are detected by monitoring the neutral current 3I 0 and the zero sequence voltage 3V 0 . Quadrilateral The quadrilateral tripping characteristic permits separate setting of the reactance X and the resistance R. The resistance section R can be set separately for faults with and without ground involvement. This characteristic has therefore an optimal performance in case of faults with fault resistance. The distance zones can be set forward, reverse or non-directional. Sound phase polarization and voltage memory provides a dynamically unlimited directional sensitivity. MHO The mho tripping characteristic provides sound phase respectively memory polarization for all distance zones. The example in figure 21 shows the characteristic for a forward fault where the mho circle expands to the source impedance but never more than the selected impedance reach. This mho circle expansion guarantees safe and selective operation for all types of faults even for close-in faults. Insensitive to measuring value distortion Digital filtering of the measured values makes the protection unit insensitive to disturbances in the measured quantities. In particular the influence of DC components, capacitive voltage transformers and frequency changes are reduced. reverse Load forwards Z3 Figure 21 Distance protection: MHO characteristic X ϕ Line Z2 Z1B α Z1 reverse Figure 20 Distance protection: quadrilateral characteristic Load X Z3 (if revers) Fuse failure monitoring The loss of the secondary V.T. circuit voltage blocks the distance protection automatically and prevents the relay from unwanted tripping. The secondary V.T. voltage is supervised by the integrated fuse failure monitor. A pickup of the fuse failure monitor as well as the ϕ Z1 Line Z2 Z4 Z1B Z5 Z4 Z5 Load forwards Load V.T.-mcb binary input block the distance protection and can activate the backup overcurrent protection. R R 12 Siemens SIP 4.2 · 08/99
Functions Fault locator The integrated fault locator calculates the fault impedance and the distance-to-fault. The result is displayed in ohms, miles, kilometers or in percent of the line length. Parallel line and load current compensation is also available. Power swing detection (ANSI 68, 68T) Dynamic occurrences, for instance short circuits, load fluctuations, autoreclosures or switching operations can cause power swings in the transmission network. During power swings, large currents along with small voltages can cause unwanted tripping of distance protection relays. To avoid uncontrolled tripping of the distance protection and to achieve controlled tripping in the event of loss of synchronism the 7SA522 relay is equipped with an efficient power swing detection function. Power swings can be detected under symmetrical load conditions as well as during single pole autoreclosures. Pilot- (Tele-) protection for distance protection (ANSI 85-21) A pilot protection function is available for fast clearance of faults up to 100% of the line length. The following operating modes may be selected: − PUTT, permissive underreaching zone transfer trip − POTT, permissive overreaching zone transfer trip − UNBLOCKING − BLOCKING − DUTT, direct underreaching zone transfer trip (together with Direct Transfer Trip function) The carrier send and receive signals are available as binary inputs and outputs and can be assigned freely to each physical relay input or output. At least one channel is required for each direction. Common transmission channels are power-line carrier, microwave radio and fiber-optic links. Figure 22 Power swing current and voltage wave forms A serial remote relay interface is in preparation for direct connection to a digital communication network 7SA522 also permits the transfer of phase-selective signals. This function requires three channels for each direction. This feature is particularly advantageous as it permits to execute reliable single-pole tripping, even if two single-phase faults occur on different lines. The transmission methods are suitable also for lines with three ends (three-terminal lines). Phase-selective transmission is also possible with multi-end applications, if some user specific linkages are implemented by way of the integrated CFC LOGIC. During disturbances in the transmission receiver or on the transmission circuit, the pilot protection function can be blocked by a binary input signal without loosing the zone selectivity. The control of the overreach zone Z1B (zone extension - can be switched over to the autoreclosure function. A transient blocking function (Current reversal guard) is provided to overcome current reversals resulting from the clearance of short circuits on parallel lines. Figure 23 Power swing circle diagram Direct transfer tripping There are various conditions on the power system which necessitate the remote tripping of the circuit breaker from the protection position. The 7SA522 relay is equipped with phase-selective intertripping signal inputs and outputs. Siemens SIP 4.2 · 08/99 13
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